In recent years, the demand for debarked logs from sawmills and wood-pulp industries has increased substantially. After the branches of a felled tree have been removed, its stem is passed through a machine for removing the bark.
One type of debarking apparatus which is widely used is of the so-called "ring type" (or rotor) through which the log is fed forward while a plurality of bark scraping tools are urged against the log surface. The tools pivot on spindles extending parallel to the axis of rotation of the ring and are spring loaded so as to keep the tips of the tools, when the machine is running idle, in the neighbourhood of the rotor axis.
The machine also comprises a log feed device which automatically holds and advances the log through the rotor. When a log is initially introduced to the machine, it is pressed against the leading edge of the debarking tools which, under the influence of the rotation of the rotor, causes each tool to open against the spring pressure and place the working tip in contact with the outside surface of the log.
As the atmospheric conditions in which these machines operate vary within wide limits, it has been observed that a satisfactory operation cannot be achieved by continuously utilizing the same spring rigidity, on the tools. For example, when the logs are in a frozen state, it is necessary to provide a certain amount of spring pretensioning for firmly pressing the tools against the log surface in order to completely remove the frozen bark. On the other hand, in the summer, a lesser amount of spring pretensioning is necessary since the bark is relatively soft.
For maintaining a satisfactory operation all year long, presently used debarking machines are provided with spring pretensioning devices; an example is given in Canadian Patent No. 675,525 issued Dec. 3, 1963 to Soderhamns Verkotader AB. This patent teaches an hydraulic spring tensioning assembly constituted by a piston sliding in a cylinder, one assembly being used with every bark scraping tool of the ring. The device, however, suffers from problems inherent to all hydraulic systems, such as leaks, complexity, excessive cost, etc. The major drawback of the tensioning device described in this patent resides in its bulkiness. Due to the cumbersomeness of the tensioning assemblies, the number of tools which may be mounted on the rotor is limited. A rotor with an opening having a diameter of 18 inches, which is a size in great demand can be provided with only five tools. It has been found highly desirable to mount an additional tool to a rotor of such size to increase substantially the debarking operation. However, the mounting of a sixth tool cannot be envisaged on 18-inch diameter rotors since there is simply not enough place for an additional piston-cylinder tensioning assembly.
It should be noted that debarking machines with larger size rotors (from 24 inches in diameter) utilizing six tools, are known. On such rotors, mounting a sixth tool is possible as the available surface for the tools and the tensioning assemblies augment by the square of the radius increase of the rotor. However, up until now, 18 inch rotors have been limited only to five bark scraping tools.
The spindles of the prior debarking machines on which the debarking tools are mounted for pivoting, are received in holes made on the rotor housing adjacent the periphery of the central opening through which the logs are passed. The rotor is usually a casting into which the spindle holes are drilled and bearing seats are machined for receiving the spindle bearings. After a certain period of time, the bearing seats wear out and must be serviced. In such case, the rotor is removed, the bearing seats surfaces remetalized and machined. This is a costly operation and involves a loss of a few production days.
For achieving a high degree of reliability and durability, present debarking machines are provided with a system supplying liquid lubricant to the rotor bearing which needs constant lubrication. A known lubrication system includes a pump continuously recirculating and spraying oil through conduits inside the machine. Another system, also in use, comprises a moving endless chain passing through an oil sump and distributing the lubricant by dripping action.
However, both these lubrication systems have not been found satisfactory from the stand point of reliability as stoppage might pass unnoticed by the machine operator with disastrous consequences. Further, in both systems, the circulated lubricant is contained in a pan into which condensation, dust and impurities accumulate, contaminating the lubricant and rendering it improper for further use.
The rotor of a debarking machine is supported on a bearing whose backlash, or free play must be adjusted from time to time to compensate for the wear. This is achieved by adjusting the pressure on the bearing exerted thereon by an adjusting ring screwed in the stator of the machine which supports the rotor. In the prior art devices, during the adjusting procedure, the ring must be rotated manually and, as it is almost flush with the stator, it offers little hold, rendering the process difficult and time consumming.
In debarking machines of the type under consideration, the rotor is rotated at a relatively high speed and the tools are continuously maintained in contact with the log surface. Therefore, most types of barks are immediately torn to small pieces and and eliminated. However, with some types of wood, the bark is removed from the log in the form of long slivers which have the tendency to adhere to the bark scraping tools impairing the machine operation.
In order to obviate this problem, it has been suggested to utilize a cleaning knife which pivots in front of the rotor transversely of the axis of the latter in the neighbourhood of the tools for cutting apart the back fiber. The clearance space between the knife and the tools must be in the order of 1/8 of an inch for achieving a satisfactory operation. An example of such cleaning knife is given in Canadian Patent No. 584,237 issued Sept. 29, 1959 to Soderhamns Verkotader disclosing, more specifically, a cleaning knife positioned by the degree of opening of the log feed device of the machine. This system, however, has not been found sucessfull and is no longer in use since the position of the knife relatively to the rotor opening depends upon the position of the log feed device, which is a function of the diameter of the log to be debarked. It has been observed that deformed logs may interfere with the knife and drag it into the plane of rotation of the tools causing extensive damages.